Optical disks called “digital versatile disk (DVD)” have been put on the market as a high-density high-capacity optical information recording medium. Such optical disks are now springing into wide use as recording mediums for recording images, music and computer data. Optical disks have a protective layer, which is considered specific to them, and therefore have such characteristics as to be tough to scratch and contamination. However, if the protective layer has a thickness error or a refractive index error, a third-order spherical aberration component of wavefront aberration occurs, which can seriously affect information recording/reproduction characteristics.
An example of a conventional optical disk will be described with reference to drawings. FIG. 12 is a diagram schematically showing an optical disk called a DVD. The optical disk 17 has a protective layer 14, a recording layer 15, and a reinforcing substrate 16. Light having a wavelength of 660 nm is converged by an objective lens 19, and the recording layer 15 is irradiated with this light on the protective layer 14 side, thereby performing recording and/or reproduction of information.
The objective lens 19 is designed so as to have a numerical aperture of 0.6 and so that a third-order spherical aberration component of wavefront aberration which occurs when light having a wavelength of 660 nm passes through a light-transmitting flat plate having a refractive index of 1.58 and a thickness of 0.6 mm is substantially zero.
Polycarbonate is used for the protective layer 14, and a film containing a dielectric or a reflecting film is used for the recording layer 15. The reinforcing substrate 16 prevents the optical disk 17 from being warped or broken.
The protective layer 14 protects the recording layer 15 against air. Also, a surface 18 of the optical disk 17 is separated from the recording layer 15 by the protective layer 14 to prevent degradation of recording or reproduction performance due to dust on the surface 18 or scratches in the surface 18.
However, if the protective layer 14 has a thickness error or a refractive index error, a spherical aberration occurs in the spot formed on the recording layer 15 to badly affect recording/reproduction of information. There is therefore a need to control the thickness and refractive index of the protective layer 14.
FIG. 13 shows an example of specified values of the refractive index and thickness of the protective layer of a DVD. The abscissa represents the refractive index of the protective layer 14, and the ordinate represents the thickness of the protective layer 14. The polygonal line in the graph indicates a region of the refractive index and the thickness with which spherical aberration is within about 30 mλrms. For example, if the design values of the refractive index and thickness of the protective layer are fixed at a certain point on the dotted line in FIG. 13 and variation in thickness is limited within the region, a disk capable of normal recording and reproduction of information can be obtained.
FIG. 14 shows a case where a sheet layer 11 and the reinforcing substrate 16 having the recording layer 15 are bonded together by an adhesive layer 13. The protective layer 14 is constituted by the sheet layer 11 formed of polycarbonate or the like and the adhesive layer 13 formed of an ultraviolet curing resin or the like. The refractive index of the sheet layer 11 at a wavelength of 660 nm is 1.58 and the refractive index of the adhesive layer 13 at the same wavelength is 1.51.
In such a case, spherical aberration occurs due to the difference between these refractive indices even if there is no error in the thickness 0.6 mm of the protective layer 14. For example, if the thickness of the sheet layer 11 is 0.56 mm and the thickness of the adhesive layer 13 is 40 μm, a spherical aberration of about 0.3 mλrms occurs, which is sufficiently small.
Thus, in the case where the numerical aperture is 0.6 and the wavelength is 660 nm, the spherical aberration that occurs due to the difference between the refractive indices of the plurality of layers constituting the protective layer is sufficiently small and can therefore be ignored. That is, in the conventional art, the protective layer constituted by a plurality of layers can be treated as one layer and it is possible to avoid an adverse effect on recording and reproduction of information by controlling the refractive index and thickness of the protective layer within the certain ranges shown in FIG. 13 in accordance with a product standard for an optical disk in which spherical aberration is limited.
In recent years, however, studies of next-generation optical disks having a higher recording density have been advanced in various regions. Next-generation optical disks are expected as a recording medium which can replace the currently dominant video tape recorders (VTRs), and the development of them is being promoted in a high pace.
As a means of increasing the recording density of optical disks, there is a method of reducing the spot formed on the recording surface. This is achieved by increasing the numerical aperture for light from the optical head and by reducing the wavelength.
This method, however, has the contrary effect of sharply increasing the spherical aberration due to a thickness error and a refractive index error of the protective layer. A need then rises to control the thickness and refractive index of the protective layer as in the case of the above-described DVD.
FIG. 15 is a diagram schematically showing an optical disk having an increased recording density. The optical disk 27 has a protective layer 24, a recording layer 25, and a reinforcing substrate 26. Light having a wavelength of 400 to 410 nm is converged by an objective lens 29, and the recording layer 25 is irradiated with this light on the protective layer 24 side, thereby performing recording and/or reproduction of information.
The numerical aperture of the objective lens 29 is large, about 0.85. Therefore two lenses are used as the objective lens 29. The objective lens 29 is designed so that a third-order spherical aberration component of wavefront aberration which occurs when light having a wavelength of 405 nm passes through a light-transmitting flat plate made of polycarbonate or the like and having a refractive index of 1.62 and a thickness of 100 μm is substantially zero. The spot formed on the recording layer 25 is reduced in size by increasing the numerical aperture and by reducing the wavelength to achieve an increase in density.
As the recording layer 25, a film containing a dielectric or a reflecting film is used. The reinforcing substrate 26 prevents the optical disk 27 from being warped or broken.
The protective layer 24 protects the recording layer 25 against air. Also, a surface 28 of the optical disk 27 is separated from the recording layer 25 by the protective layer 24 to prevent degradation of recording or reproduction performance due to dust on the surface 28 or scratches in the surface 28.
A sheet layer 21 and the reinforcing substrate 26 having the recording layer 25 are bonded together by an adhesive layer 23. The protective layer 24 is thus formed of two layers.
The sheet layer 21 is formed of polycarbonate or the like and the adhesive layer 23 is formed of an ultraviolet curing resin or the like. The refractive index of the sheet layer 21 at a wavelength of 405 nm is 1.62 and the refractive index of the adhesive layer 23 at the same wavelength is 1.53. In such a case, spherical aberration occurs due to the difference between these refractive indices even if there is no error in the design thickness 100 μm of the protective layer 24.
For example, even if the thickness of the sheet layer is 60 μm; the thickness of the adhesive layer 23 is 40 μm; and the thickness of the protective layer 24 is 100 μm, a spherical aberration of 5.3 mλrms occurs. This spherical aberration remains initially as residual aberration.
Apart from this, a spherical aberration due to variation in thickness which occurs in manufacture of the optical disk is also added. Ordinarily, a thickness variation of about 3 μm occurs and a spherical aberration of 30 mλrms due to the thickness variation results.
Consequently, even if the thickness of the protective layer 24 can be adjusted to 100 μm, the total spherical aberration including the above-mentioned residual spherical aberration 5.3 mλrms is 35.3 mλrms and normal recording or reproduction cannot be performed.
FIG. 16 shows comparison between spherical aberration due to the adhesive layer when the numerical aperture is 0.6 and the wavelength is 660 nm (DVD) and spherical aberration due to the adhesive layer when the numerical aperture is 0.85 and the wavelength is 405 nm. As can be understood from this graph, a large spherical aberration is caused due to the different refractive indices of the plurality of layers constituting the protective layer when the numerical aperture is large and the wavelength is short. This aberration is 15 times or more larger than that in the case of the DVD. If a variation in thickness of the protective layer is further added, the acceptable limit spherical aberration is exceeded, resulting in failure to perform normal recording or reproduction.